Gamma cameras typically are used for locating and displaying abnormalities in human glands and organs. More specifically, and with respect to using a gamma camera, gamma-ray-emitting tracer material is administered to a patient, and the tracer material is more greatly absorbed by the abnormality to be detected than by the other tissues. The gamma camera generates data, or an image, representing the distribution of such tracer material within the patient.
A conventional gamma camera includes a collimator and a scintillation crystal, or detector, responsive to radiation stimuli, i.e., gamma rays emitted by the patient. The collimator is positioned adjacent one face of the crystal, and includes a collimator core fabricated from gamma ray attenuating material and having a plurality of openings. An array of photomultiplier tubes typically are positioned adjacent an opposite face of the crystal.
In operation, the gamma rays emitted by the patient are projected toward the collimator core, and those rays projecting through the collimator openings interact with the crystal. The gamma rays impinging upon the collimator septa, i.e., impinging upon the attenuating material and not projecting through the collimator openings, are substantially attenuated and do not interact with the crystal.
Light events occur within the crystal at locations where the rays interact with the crystal lattice structure. The photomultiplier tubes, in response to the light events, produce individual analog outputs. In digital gamma cameras, the analog photomultiplier tube outputs are supplied to analog-to-digital converters (ADCs) which convert the analog outputs to digital signals.
To generate an image, a representation of the distribution of events in the crystal is generated by utilizing a matrix of storage registers whose elements are in one-to-one correspondence with elemental areas of the crystal. The crystal elemental areas are identified by coordinates. Each time a light event occurs in the crystal, the event coordinates are identified and the register in the storage register matrix corresponding to the identified event coordinates is incremented. The contents of a given register in the matrix is a number that represents the number of events that have occurred within a predetermined period of time within an elemental area of the crystal. Such number is directly proportional to the intensity of radiation emitted from an elemental area of the radiation field. The number stored in the register therefore is used to establish the brightness of a display picture element corresponding to the crystal elemental area. The distribution of a radiation field is displayed in terms of the brightness distribution of the display.
Gamma cameras sometimes are used in connection with ultra-high energy isotopes such as F-18. With the higher energy isotopes, improved images can be generated for certain body parts such as the brain and the heart. Although higher energy isotopes facilitate generation of improved images, the gamma rays generated using such high energy isotopes penetrate through known gamma camera collimators. Particularly, the higher energy gamma rays penetrate through the collimator septa and interact with the scintillation crystal. Therefore, rays which normally would be blocked at lower energies generate light events and thus reduce image contrast and diagnostic image quality.
To reduce such collimator penetration caused by high energy isotopes, collimators have been modified to include thicker collimator cores. Although such thicker collimators reduce gamma ray penetration from high energy isotopes, such collimators also weigh substantially more than collimators suitable for lower energies. With this increased weight, the collimator may exceed the weight bearing capacity of the gamma camera or other nuclear imaging system components such as a collimator cart or exchange system. In addition, changing collimators for different imaging sessions is more cumbersome with heavier collimators.
Accordingly, it would be desirable to provide a collimator which both reduces gamma ray penetration from ultra-high energy isotopes and does not weigh significantly more than typical collimators. It also would be desirable to provide such a collimator which does not significantly degrade image quality.